Processing of steam-cracked naphtha light end products



iinitsd States Patent i PROCESSING OF STEAM-CRACKED NAPHTHA LIGHT END PRODUCTS Robert H. Johns'ton, Whippany, and Isaac Pass, Roselle,

N J., assrgnors to Esso Research and Engineering Comparty, a corporation of Delaware Filed June 9, 1958, Ser. No. 740,765

7 Claims. (Cl. 208-104) This invention relates to a means and method for processmg low-boiling unsaturated hydrocarbon products separated from a quenched steam-cracked naphtha. It is concerned particularly with obtaining a more complete segregation of ethene, propene, butene, butadiene, and pentadiene (C to C concentrates from the more readily condensed higher boiling hydrocarbon products by keepmg temperature, pressure, and time of separation steps at low levels to minimize polymerization of the reactive olefinic hydrocarbons in the C to C range.

To obtain quick separation of the C to C hydrocarbons at low temperature and pressure levels, a splitter tower is used on the lowest pressure side of a series of compressors and coolers for removing up through C hydrocarbons from higher boiling hydrocarbons of an initial naphtha range condensate. Since the fractionation is not precise, the overhead vapors from the stripping zone are Patented June 28,

In the drawing is shown diagrammatically. a'flow plan illustrating the arrangement of the splitter tower and the compressors with respect to the steam-cracking coil, the quench tower and its overhead cooling and condensate receiving means.

Referring to the drawing, the naphtha fraction used as cracking feed starts from supply source 1 to flow by line 2 through the cracking coil 3 in furnace 4. The steam is admixed by line 5. The high-temperature steam-cracking products leave the outlet of the cracking coil through transfer line 6 to be passed into a bottom part of quenching zone in tower 7. Quench oil is injected into the transfer line 6 from line 8. At least a substantial portion of the quench oil passing through line 8 is supplied from an external source by line 9, this external source preferably supplying a light gas oil which is refractory to I cracking and at least has a low susceptibility to the formation of tarry polymers which would coke up the transfer line 6. The light gas oil may be obtained by relatively low-temperature cracking of gas oil, e.g., low temperature catalytic cracking, or maybe a virgin light gas oil. Prefmainly C to C hydrocarbons with small amounts of higher boiling hydrocarbons.

I In steam cracking, the hydrocarbon vapors of-a virgin steam to form a cracking mixture containing 60 to- 90' mole percent steam which is heated in a cracking coil under a total pressure of generally 1 to 3 atmospheres for a cracking period of a fraction of. aminute.

The cracked products are transferred from the outlet of the steam-cracking coil to a quench tower bya transfer line. In the transfer line the cracked mixture is commingled with a sufiicient amount of relatively cool quench oil to reduce a temperature in the range of 135 0 F. to 1500 F. to a temperature in a range of 400 F. to 650 F. as the mixture is passed into a bottom part of the quench tower. In the quench tower, most of the fur-- ther cooling is obtained by introducing large amounts of high-boiling hydrocarbon liquids into the upper part of the tower or quenching zone. so as to flow countercurrently to the gaseous stream which flows rapidly up through and out overhead from the tower. contains cracked hydrocarbon products boiling below 430 F. mixed with a higher proportion of steam and some hydrogen and methane.

The overhead gaseous stream from the quench tower is passed to a heat exchange cooling condenser to condense out water and mainly hydrocarbons higher boiling than C hydrocarbons. The water is separated in a separation zone and a portion of the condensed hydrocar bons may be used as reflux to the upper part of the quench tower. The remaining part of the condensed hydrocarbons from the overhead separated from water in .said separation zone is fed to the fractionation zone of a splitter tower for removal therefrom. of desired vaporized butenes, butadienes, pentenes, and pentad-ienes (C and lighter), which are to be recovered as light end concentrates. a

The operation of the splitter tower in conjunction with. the remaindertof the steam-cracking unit and its light ends recovery system will be described in more" detail with reference to'the' attached drawing.

This gaseous stream erably, the gas oil should be aromatic in'order to be more refractory to. cracking. This light gas oil can'be' added in suitable amounts to a portion of bottoms withdrawn from the quench tower 7 through line lthpump 11, line 12 and cooler 13. In combining the gas oil with the recycled cooled bottoms, a suflicient amount of the gas oil is admixed to keep the tarry polymer content of the combined mixture at a suitable low level, e.g., below 10%.

Another portion of the heavy fraction bottoms with drawn from tower 7 through line It) and pump 11, is passed through a cooler 14- for recycling to an upper part of quench tower 7. The cooler 14 extracts as much heat as practical to bring the temperature of the bottoms passed there-through down to below 300 F. A portion of the bottoms ,is purged through line 15 which branches from line 16 in order to prevent build up of tar. Another por cracked light ends in making a separation of ethylene. This sponge oil thus used and enriched isreturned to the 7 upper part of quench tower 7 through line 18. An additional cooling liquid stream may be obtained by refluxing condensate from the tower overhead through line 19 if desired.

The overhead vapors are taken from quench tower 7 at a temperature in the range of 2.00 to 300 F. and a pressure in the range of 1 to 3 atmospheres (14.7 to 58.8

p.s.i.a.)' throughline 20 into cooling condenser 21 whence."

condensate and gaseous products are passed into the separation tank 22. A bottom water layer settled m .tank 22 is withdrawn through line 23. A portion of the hydrocarbon oil condensate withdrawn from an upper liquid oil layer in tank 22 through line 24 may be returned to tower 7 by line 19 as reflux. The remainlng Oll condensate withdrawn by line 24 is passed by line 25 into the splitter tower 26.

The splitter tower 26 is a fractronatmg zone colurnn' provided with plates or similar liquid-vapor contacting means for obtaining separation of volatlle hydrocarbons in the C and lighter range which are taken overhead through line 27. At the bottom of splitter tower 26 is" provided a reboiler 28 for supplyingheah Residual oil is withdrawn from tower 26 through line 29. T1115 residual oil contains principally C and higher boiling liquid hydrocarbons and these liquid hydrocarbons may. I

then be subjected to conventional refining treatments,

3 efficient separation. pors of tower 7 and condensate from the overhead vapors of the splitter tower 26 are combined in tank 22.

From receiving tank 22 the uncondensed hydrocarbon vapors are drawn through line 30 to the suction side of a first stage compressor 31 and the thus compressed vapors are passed byline 32 through cooling condenser 33 into a first separation drum 34. In drum 34 some condensate of C and higher hydrocarbons is collected as a bottom liquid phase to be withdrawn by line 35 so as to join with the liquid being passed by line 25 into the splitter tower 26. Uncondensed vapors are drawn from the upper part of drum 34 through line 36 by the second compressor 37 which compresses the vapors and sends them through line 38 and cooler 39 into a second separating drum 40. In drum 40 the liquid condensate collected at the bottom contains mainly liquid C and C hydrocarbons. These liquid hydrocarbons are withdrawn by line 4-1. Vapors are drawn from the upper part of drum 49 through line 42 by the third compressor 43 which compresses the vapors and sends them through line 44 and cooling condenser 45 into a third separating drum 46. To aid in the cooling, liquid withdrawn from drum 40 through line 41 is admixed with the compressed vapors in line 44-. In drum 46 another liquid from vapor separation may be made so that liquid and vapor can be treated separately. Liquid from drum 46 is passed by line 47 to a desulfurizing treatment unit 48. Vapor is withdrawn from drum 46 by line 49 to a similar desulfurizing treating unit 48. This last stage of vapor from liquid separation is carried out for the sake of flexibility in treating the vapor and the liquid separately, if so desired, before the naphtha hydrocarbons in the streams are passed finally into a first absorber 50 as by line 51.

The first absorber 50 receives the lean light naphtha in a lower part and a liquid oil as solvent from line 52 in the upper part of the column so that this latter oil can be passed downwardly countercurrent to gaseous .C and C hydrocarbons for absorbing out higher boiling components. The unabsorbed gases leave overhead from absorber 50 and are passed through line 53 to the second absorber 54, called a sponge oil absorber. This absorber receives at its upper part through line 54: a lean absorption oil, e.g. a heavy oil or the bottoms fraction taken from the quench tower 7 through line 17. In the sponge oil absorber this heavy oil functions to remove hydrocarbons containing more than two carbon atoms per molecule from the gas rich in ethylene which leaves overhead from the absorber 54 through line 55. The sponge oil enriched by absorbed C and higher hydrocarbons is withdrawn from the bottom of absorber tower 54 through line 56 to be returned to the quench tower 7 through line 18, as previously mentioned. In this sponge oil extraction of higher hydrocarbons from the ethylenerich gases, some of the higher olefinic and diolefinic hydrocarbons become extracted but are then recovered since they are returned with the sponge oil to the quench tower 7.

The light naphtha fraction rich in absorbed C and C hydrocarbon components is withdrawn from the bottom of the first absorber tower 50 through line 57 to be passed to subsequent fractionating columns, e.g., the debutanizer 53. In debutanizer 5% the propylene-rich fraction is taken overhead through line 59'for transfer to a depropanizer. The debutanized naphtha containing mainly C hydrocarbons stripped of the propylene is withdrawn from the debutanizer 58 through line 60. This lean C rich hydrocarbon oil boiling mainly in the range of 80 F. to 200 F. can be divided to send a portion through line 61 back to line 52 to enter the upper part of the first absorber. The remaining divided portion of the lean naphtha. oil can be sent through line 62 to an- Condensate from the overhead va- 4 aratiou of a C fraction rich in ethylene, and also C and C fractions rich in propylene and butadiene with minimum exposure to high temperatures so as to prevent excessive polymerization with its attendant difliculties and lowering of yield.

The compressors used in stages with intercoolers build up the pressure without extensive increase in tempera ture. A low boiling naphtha is used as solvent in the absorber 50. This solvent containing mainly C hydrocarbons is made to flow down through the first stage absorber at a rate of about 0.7 to 1.0 mole per mole of hydrocarbon fed from line 51 and under conditions such that the C rich overhead is at a temperature of 60 to 75 F. and temperatures are no higher than about 320 F. down to the bottom of absorber tower 50. In the second absorber 54 temperatures of 50 to 120 F. and pressures essentially the same as absorber 50 are used.

Examples of preferred conditions and advantages with regard to the operation of the splitter tower are given as follows.

The splitter tower 26 is preferably operated as a selfreliuxing type rather than as a pump back refluxing type fractionating column. In the self-refluxing tower, which is shoxm in the drawing, a partial condensation of the overhead vapors is carried out, the resulting condensate becomes admixed with fresh feed to the tower, and the feed to the tower is supplied to a top plate of the tower. Here theprcssure is close to 2 atmospheres and the temperature is close to 180 F. At the bottom of the splitter may be made substantially free of C and lighter material.

absorber and in the debutanizer.

An important function of the splitter tower 26 is to separate as much as possible of the C and higher bydrocarbons from the gaseous stream which is passed to the compression stages and then to the absorber-deethanizer in order to avoid excessive temperatures in the When the bottoms temperatures in the absorber and debutanizer exceed 320 F. in bringing the bottoms to the bubble point, excessive polymerization takes place in these towers. With an increase of hydrocarbons higher boiling than C hydrocarbons, the bubble point at the bottom of these towers tends to increase above 320 F. Furthermore, by satisfactory removal of the C and higher hydrocarbons in the bottoms of the splitter tower, the circulation of the lean absorbent oil used in the absorber is decreased in total flow quantity. At the same time this lean oil becomes more concentrated in C components.

Since the splitter tower operates at a low pressure, it is able to fractionate without excessive temperatures, and similarly at a bubble point no higher than 320 F. in the bottoms. The arrangement of the separator tower so that its vapors go to the first compressor permits the operation of the splitter tower at the desired low pressures below 5 atmospheres, and preferably about 3 atmospheres or less.

The uncondensed vapors from the splitter tower and from the quenching zone overhead, separation zone are desired.

The arrangement of apparatus and steps described isparticularly useful for obtaining quick and sharp sepsuitable for compression. Several compression stages are used in order to avoid exceeding a temperature of 250 F. in bringing the vapors up to a pressure of 200 p.s.i.g. or between 13 and 16 atmospheres, which pressure is needed in the subsequent absorber-deethanizer tower 50.

The invention described is claimed as follows:

1. In a process for recovering C to C5 hydrocarbon fractions rich in alkenes and dienes from a gaseous stream including steam, C to C hydrocarbons and higher boiling naphtha hydrocarbons separated overhead from higher boiling steam-cracked naphtha products in a quenching zone, the improvement which comprises passing said gaseous stream to condensing and separation zones; con-' densing water with C and higher boiling hydrocarbons from said gaseous stream, separating condensed hydrocarbons from the water and from uncondensed gaseous hydrocarbons in the separation zone, passing these uncondensed gaseous hydrocarbons to a vapor compression zone, passing thus separated condensed hydrocarbons into a fractionation zone, and passing at least a substantial part of the vapors rich in C and lighter hydrocarbons from said fractionation zone to join the gaseous stream from the quenching zone prior to condensation thereof, to obtain cooling of said gaseous stream and an increased amount of condensed hydrocarbons in said separation zone.

2. The process of claim 1 wherein said condensed hydrocarbons separated from water in said separation zone being passed into a top part of said fractionation zone for quick stripping of light components.

3. The process of claim 1 wherein said condensed liquid hydrocarbons separated from water in said separation zone being passed to a plate near the top of said fractionation zone so as to obtain quick stripping of light components, said fractionation zone being of the type in which no part of the overhead vapors from the zone are externally condensed, separated from vapors and recycled as a reflux stream to the top of the distillation zone.

4. The process of claim 1 in which the entire overhead vapor stream from the fractionation zone is passed back to join the gaseous stream from the quenching zone.

5. In the process of claim 1, gaseous hydrocarbons drawn from said separation zone into the compression 6 zone are compressed and cooled to obtain an additional condensate of hydrocarbons higher boiling than C hydrocarbons, and said condensate is passed into said fractionating zone.

6. In the process of claim 1, said gaseous stream withdrawn from said separation zone to the vapor compression zone is compressed and then cooled to form a compressed gaseous stream richer in ethene, after which said ethene-rich gaseous stream is contacted with a condensate from the steam-cracked naphtha hydrocarbons rich in C dienes and other hydrocarbons boiling principally in the range of to 200 F. to absorb C and higher boiling hydrocarbons from the ethene-rich gaseous stream, subsequently said ethene-rich gaseous stream is contacted with a cooled portion of the higher boiling steam-cracked naphtha product withdrawn as a liquid residual product from said quenching zone to extract from the ethenerich vapor stream higher boiling components.

7. The process as in claim 6, wherein said cooled portion of the higher boiling steam-cracked naphtha products containing components. extracted from said ethenerich vapor stream is returned to an upper part of said quenching zone.

References Cited in the file of this patent UNITED STATES PATENTS Johnston et a1 May 15, 1956 

1. IN A PROCESS FOR RECOVERING C2 TO C5 HYDROCARBON FRACTIONS RICH IN ALKENES AND DIENES FROM A GASEOUS STREAM INCLUDING STEAM, C2 TO C5 HYDROCARBONS AND HIGHER BOILING NAPHTHA HYDROCARBONS SEPARATED OVERHEAD FROM HIGHER BOILING STEAM-CRACKED NAPHTHA PRODUCTS IN A QUENCHING ZONE, THE IMPROVEMENT WHICH COMPRISES PASSING SAID GASEOUS STREAM TO CONDENSING AND SEPARATION ZONES, CONDENSING WATER WITH C4 AND HIGHER BOILING HYDROCARBONS FROM SAID GASEOUS STREAM SEPARATING CONDENSED HYDROCARBONS FROM THE WATER AND FROM UNCONDENSED GASEOUS HYDROCARBONS IN THE SEPARATION ZONE PASSING THESE UNCONDENSED GASEOUS HYDROCARBONS TO A VAPOR COMPRESSION ZONE, PASSING THUS SEPARATED CONDENSED HYDROCARBONS INTO A FRACTIONATION ZONE, AND PASSING AT LEAST A SUBSTANTIAL PART OF THE VAPORS RICH IN C5 AND LIGHTER HYDROCARBONS FROM SAID FRACTIONATION ZONE TO JOIN THE GASEOUS STREAM FROM THE QUENCHING ZONE PRIOR TO CONDENSATION THEREOF, TO OBTAIN COOLING OF SAID GASEOUS STREAM AND AN INCREASED AMOUNT OF CONDENSED HYDROCARBONS IN SAID SEPARATION ZONE. 